The human immune system is comprised of cellular and molecular components designed to coordinately prevent infection while avoiding potentially harmful inflammation and auto-immunity. Immunity varies with age, reflecting unique age-dependent challenges including fetal gestation, the neonatal phase and infancy. Herein, we review novel mechanistic insights into early life immunity, with emphasis on emerging models of human immune ontogeny, which may inform age-specific translational development of novel anti-infectives, immunomodulators and vaccines.